Layered radiation-sensitive materials with varying sensitivity

ABSTRACT

A method for fabricating a radiation-cured structure is provided. The method includes the steps of providing a first radiation-sensitive material and applying a second radiation-sensitive material to the first radiation-sensitive material. The first radiation-sensitive material has a first sensitivity. The second radiation-sensitive material has a second sensitivity different from the first sensitivity. At least one mask is placed between at least one radiation source and the first and second radiation-sensitive materials. The mask has a plurality of substantially radiation-transparent apertures. The first and second radiation-sensitive materials are then exposed to a plurality of radiation beams through the radiation-transparent apertures in the mask to form a first construct in the first radiation-sensitive material and a second construct in the second radiation-sensitive material. The first construct and the second construct cooperate to form the radiation-cured structure.

FIELD OF THE INVENTION

The present disclosure relates to radiation-cured materials and moreparticularly to a method for fabricating radiation-cured materials withcomplex structures.

BACKGROUND OF THE INVENTION

Radiation-cured microstructures have been described by Jacobsen et al.in “Compression behavior of micro-scale truss structures formed fromself-propagating polymer waveguides”, Acta Materialia 55, (2007)6724-6733, the entire disclosure of which is hereby incorporated hereinby reference. One method and system of creating polymer materials withordered microtruss structures is disclosed by Jacobsen in U.S. Pat. No.7,382,959, the entire disclosure of which is hereby incorporated hereinby reference. The system includes at least one collimated light sourceselected to produce a collimated light beam; a reservoir having aphoto-monomer adapted to polymerize by the collimated light beam; and amask having at least one aperture and positioned between the at leastone collimated light source and the reservoir. The at least one apertureis adapted to guide a portion of the collimated light beam into thephoto-monomer to form the at least one polymer waveguide through aportion of a volume of the photo-monomer. Microtruss materials producedby the method and system are further disclosed by Jacobsen in U.S.patent application Ser. No. 11/801,908, the entire disclosure of whichis hereby incorporated herein by reference. A polymer material that isexposed to radiation and results in a self-focusing or self-trapping oflight by formation of polymer waveguides is also described by Kewitschet al. in U.S. Pat. No. 6,274,288, the entire disclosure of which ishereby incorporated herein by reference.

Products formed by bilayer resist processes have also been described,for example, by Orvek et al. in U.S. Pat. No. 4,770,739, the entiredisclosure of which is hereby incorporated herein by reference. A firstresist material sensitive to near UV or violet light is deposited overthe top surface of a body. A second resist material sensitive to deep UVlight is deposited over the first resist material. The second resistmaterial is exposed to patterned illumination of deep UV light, and thenexposed areas removed. The first resist material is illuminated by aflood or blanket exposure of near UV or violet light. The bilayer resistproduct is thereby formed.

Further known methods for fabricating microstructures include rapidprototyping technology, such as stereolithography, fused depositionmodeling, and LIGA (a German acronym for Lithography, Electroplating,and Molding). A particular rapid prototyping technology formanufacturing microstructures is known as electrochemical fabrication,for example, EFAB™ developed by Microfabrica Inc. located in Van Nuys,Calif. The electrochemical fabrication process typically begins bydepositing a sacrificial material onto a blank substrate in a desiredpattern. The sacrificial material supports the microstructure, likescaffolding, during the fabrication process. A structural material isthen deposited onto the sacrificial material. The sacrificial andstructural materials are then precisely planarized, and the processrepeated until the microstructure is fully assembled. The sacrificialmaterial is ultimately removed, for example, by a highly selectiveetching procedure to leave the completed microstructure formed from thestructural material. The use of electrochemical fabrication facilitatesthe manufacturing of microstructures with an extraordinary level ofgeometrical complexity, including the ability to create assemblies outof separate, independently-formed components. However, electrochemicalfabrication and other conventional rapid prototyping methods areundesirably expensive and time consuming, particularly for applicationssuch as automotive fuel cells.

There is a continuing need for a method for fabricating radiation-curedstructures that is less expensive and time consuming in comparison toconventional rapid-prototyping methods. Desirably, the methodfacilitates the cost-effective formation of radiation-cured componentsfor fuel cell and other applications.

SUMMARY OF THE INVENTION

In concordance with the instant disclosure, a method for fabricatingradiation-cured structures that is less expensive and time consuming incomparison to conventional rapid-prototyping methods, and thatfacilitates the cost-effective formation of radiation-cured fuel cellcomponents for fuel cell and other applications, is surprisinglydiscovered.

In a first embodiment, a method for fabricating a radiation-curedstructure includes the steps of providing a first radiation-sensitivematerial, and applying a second radiation-sensitive material to thefirst radiation-sensitive material. The first radiation-sensitivematerial has a first sensitivity. The second radiation-sensitivematerial has a second sensitivity different from the first sensitivity.At least one mask is placed between at least one radiation source andthe radiation-sensitive materials. The mask has a plurality ofsubstantially radiation-transparent apertures formed therein. The firstand second radiation-sensitive materials are then exposed to a pluralityof radiation beams through the radiation-transparent apertures in the atleast one mask to form a first construct in the firstradiation-sensitive material and a second construct in the secondradiation-sensitive material. The first construct and the secondconstruct cooperate to form the radiation-cured structure.

In another embodiment, a method for fabricating a radiation-curedstructure, includes the steps of: providing a first radiation-curablematerial, the first radiation-curable material having a firstsensitivity including at least one of a first curing rate, a firstinitiation rate, a sensitivity to a first radiation frequency, asensitivity to a first radiation amplitude, and a sensitivity to a firstradiation type; applying a second radiation-curable material to thefirst radiation-curable material, the second radiation-curable materialhaving a second sensitivity including at least one of a second curingrate, a second initiation rate, a sensitivity to a second radiationfrequency, a sensitivity to a second radiation amplitude, and asensitivity to a second radiation type, the second sensitivity differentfrom the first sensitivity; placing at least one mask between at leastone radiation source and the first and second radiation-curablematerials, the mask having a plurality of substantiallyradiation-transparent apertures formed therein; and exposing the firstand second radiation-curable materials to a plurality of radiation beamsthrough the radiation-transparent apertures in the mask to form a firstconstruct in the first radiation-curable material and a second constructin the second radiation-curable material. The first construct and thesecond construct cooperate to form the radiation-cured structure.

In a further embodiment, a method for fabricating a radiation-curedstructure, includes the steps of: providing a firstradiation-dissociable material, the first radiation-dissociable materialhaving a first sensitivity including at least one of a firstdissociation rate, a sensitivity to a first radiation frequency, asensitivity to a first radiation amplitude, and a sensitivity to a firstradiation type; applying a second radiation-dissociable material to thefirst radiation-dissociable material, the second radiation-dissociablematerial having a second sensitivity including at least one of a seconddissociation rate, a sensitivity to a second radiation frequency, asensitivity to a second radiation amplitude, and a sensitivity to asecond radiation type, the second sensitivity different from the firstsensitivity; placing a mask between an at least one radiation source andthe first and second radiation-dissociable materials, the mask having aplurality of substantially radiation-transparent apertures formedtherein; and exposing the first and second radiation-dissociablematerials to a plurality of radiation beams through theradiation-transparent apertures in the mask to form a first construct inthe first radiation-dissociable material and a second construct in thesecond radiation-dissociable material. The first construct and thesecond construct cooperate to form the radiation-cured structure.

DRAWINGS

The above, as well as other advantages of the present disclosure, willbecome readily apparent to those skilled in the art from the followingdetailed description, particularly when considered in the light of thedrawings described herein.

FIG. 1 is a schematic flow diagram of a method for fabricating aradiation-cured microstructure according to one embodiment of thepresent disclosure, showing formation of the radiation-curedmicrostructure from radiation-sensitive materials with differentsensitivities;

FIG. 2 is a schematic flow diagram of a method for fabricating aradiation-cured microstructure according to another embodiment of thepresent disclosure, showing formation of the radiation-curedmicrostructure from radiation-curable materials with differentsensitivities; and

FIG. 3 is a schematic flow diagram of a method for fabricating aradiation-cured microstructure according to a further embodiment of thepresent disclosure, showing formation of the radiation-curedmicrostructure from radiation-dissociable materials with differentsensitivities.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description and appended drawings describe andillustrate various embodiments of the invention. The description anddrawings serve to enable one skilled in the art to make and use theinvention, and are not intended to limit the scope of the invention inany manner. In respect of the methods disclosed, the steps presented areexemplary in nature, and thus, are not necessary or critical.

As shown in FIG. 1, the present disclosure includes a method 100 forfabricating a radiation-cured structure from at least a firstradiation-sensitive material and a second radiation-sensitive material.The radiation-cured structure is formed from a plurality ofradiation-cured constructs that are individually formed in each of thefirst and second radiation-sensitive materials from a plurality ofradiation-cured elements or construct features. The radiation-curedconstructs formed in each of the first and second radiation-curedmaterials cooperate to form the radiation-cured structure.

The method 100 may first include the step of providing 102 a substrate.The substrate may be formed from any material that permits the formationof polymeric structures thereon. In certain embodiments, the substrateis a substantially planar sheet. A skilled artisan should appreciatethat the substrate may be shaped, however, to provide theradiation-cured structure having the desired shape. The substrate may beelectrically nonconductive such as plastic, or electrically conductivesuch as stainless steel. The substrate may have holes formed thereinthat facilitate a removal of excess uncured radiation-sensitive materialfollowing the fabrication of the radiation-cured structure.

The substrate may further be provided with a coating or surfacetreatment for bonding and debonding from the radiation-cured structure.As a nonlimiting example, the substrate may have a coating adapted tobond with the uncured radiation-sensitive material. The surfacetreatment may further facilitate a debonding of a cured polymer from thesubstrate. In particular, a backside of the substrate is disposed on aflat surface or a stationary base plate during fabrication of theradiation-cured structure and has a coating to militate against anundesired contamination or plating of the substrate, for example, withrepeated use. The stationary base plate may be a porous vacuum chuckhaving a pressure-facilitated release, for example, for selectivelyholding the substrate in place during the fabrication process. A skilledartisan may select suitable surface treatments, including coatings, asdesired.

The method 100 further includes the step of applying 104 the firstradiation-sensitive material to the substrate, and the step of applying106 the second radiation-sensitive material to the firstradiation-sensitive material. It should be appreciated that the firstradiation-sensitive material may be provided, for example, as a freestanding film with no substrate in lieu of the step of providing 102 thesubstrate as described hereinabove. The step of applying 104, 106 thefirst and second radiation-sensitive materials may alternatively includeprelaminating the first and second radiation-sensitive materials andapplying the prelaminated first and second radiation-sensitive materialsto the substrate.

The radiation sensitive materials according to the present disclosureinclude radiation-curable materials and radiation-dissociable materials.The term “radiation-curable material” is defined herein as any materialthat is at least one of initiated, polymerized, and crosslinked byexposure to radiation. It should be appreciated that an increase intemperature may also be employed to at least partially completepolymerization or crosslinking of the radiation-curable materialsfollowing an initiation by the exposure to radiation. The term“radiation-dissociable material” is defined herein as any material thatexhibits at least one of a scissioning of the polymer backbone and adecrosslinking by exposure to radiation. As a nonlimiting example, theradiation-dissociable material may be made solvent-soluble by asufficient breakage of crosslinks and/or scissioning of the polymerbackbone of the radiation-dissociable material.

As nonlimiting examples, the radiation-curable materials may include oneof a liquid photomonomer and a substantially solid radiation-curablepolymer. The liquid photomonomer may be a monomer as described byJacobsen in U.S. Pat. No. 7,382,959 and U.S. application Ser. No.11/801,908. Further nonlimiting examples of suitable photomonomersinclude monomers that polymerize via free-radical polymerization whenexposed to UV radiation (wavelength between about 250 nm and about 400nm). The photomonomer may include any suitable free-radical photopolymermaterial such as urethanes (polyurethanes), acrylates, methacrylates,and cationic polymers such as photo-cured epoxies. Suitable liquidphotomonomers may exhibit a shift in index of refraction uponphotopolymerization, for example, to provide self-propagatingwaveguides. Other photomonomers may also be employed, as desired.

Suitable substantially solid radiation-curable polymers may includenegative resist polymers. Negative resist polymers go through aphotoinitiation process that leads to a curing of the negative resistpolymer by polymerization or polycondensation, for example. If thepolymerization or polycondensation reaction occurs at substantially thesame time, the process is referred to as “photocured”. If only thereaction species are generated by the photoinitiation process and asubsequent step such a heating is required to generate thepolymerization or polycondensation, the process is be referred to as“photoinitiated”. It should be appreciated that even though a post-cureheat treatment may be necessary to finalize the polymerization step,substantially stable construct features in the negative photoresistpolymer may also be created during the initial radiation exposure. Thesubstantially solid radiation-curable polymers can go through just theinitiation process and, due to inherent stability and the limiteddiffusion rate of the chemical species within the solidradiation-curable polymers, the curing process may also be performedmuch later without significant feature degradation. It should beappreciated that most photoinitiated polymers begin the curing processat the inception of the initiation process, but the kinetics of thereaction at the exposure temperature are so slow that little, if any,polymerization or polycondensation may take place prior to heating thenegative resist polymer to a desired curing temperature.

One particular negative resist polymer is the epoxy-based SU-8 2000™,commercially available from Microchem Corporation in Newton, Mass. TheSU-8 2000™ negative resist polymer is curable by UV radiation. It shouldbe appreciated that other substantially solid radiation-curable polymersmay also be employed.

As a nonlimiting example, the radiation-dissociable materials mayinclude positive resist polymers. Positive resist polymers begin ascrosslinked polymers but may contain photoinitiators that, when exposedto a particular radiation, generate chemical species which dissociatethe polymer by at least one of breaking the crosslinks and scissioningthe polymer backbone. The dissociation makes the positive resist polymersoluble in the regions which have been exposed to the radiation. Regionswhere the positive resist polymer remains are masked rather than beingexposed, as is the case with the negative resist polymers describedhereinabove. In certain embodiments, the positive resist polymers aresensitive to radiation, e.g., ultraviolet or electron beam, without theneed for photoinitiators. For example, the positive resist polymer mayitself be damaged by the radiation and the remaining scissioned chainsbecome soluble in a solvent. Other types of positive resist polymers maybe employed, as desired.

The first radiation-sensitive material has a first sensitivity. Thesecond radiation-sensitive material has a second sensitivity differentfrom the first sensitivity. As disclosed herein, the sensitivity withrespect to radiation-curable materials is at least one of a curing rate,an initiation rate, a sensitivity to radiation frequency, a sensitivityto radiation amplitude, and a sensitivity to radiation type. Thesensitivity with respect to radiation-dissociable materials is at leastone of a dissociation rate, a sensitivity to radiation frequency, asensitivity to radiation amplitude, and a sensitivity to radiation type.It should be appreciated that upon applying 104, 106 the first andsecond radiation-sensitive materials to the substrate, differentradiation-cured constructs may be formed in the first and secondradiation-sensitive materials by exposing 110 the first and secondradiation-sensitive materials to radiation selected for the respectivefirst and second sensitivities. Radiation-cured structures having a highlevel of geometrical complexity may thereby be fabricated.

It should further be understood that use of the firstradiation-sensitive material having the first sensitivity, and thesecond radiation-sensitive material sharing the first sensitivity butalso having the second sensitivity different from the first sensitivity,is within the scope of the present disclosure.

In a further embodiment, the method 100 may include the step of applyinga third radiation-sensitive material to the second radiation-sensitivematerial. The third radiation-sensitive material has a thirdsensitivity. The third sensitivity may be the same as or different fromone of the first and second sensitivities, as desired. In one example,the third sensitivity is substantially the same as the firstsensitivity. In a particular example, the third sensitivity is differentfrom the first and second sensitivities. In a most particular example,the third sensitivity is different from the second sensitivity andsubstantially the same as the first sensitivity. One of ordinary skillin the art should appreciate that a resulting radiation-cured structurecould thereby be formed having layers of substantially the sameconstruct with a layer of a different construct disposed therebetween.It should also be appreciated that any desired number ofradiation-sensitive materials in any desired arrangement, for example,laminated or otherwise, may be employed within the scope of the presentdisclosure.

Following the steps of applying 104, 106 the first and secondradiation-sensitive materials to the substrate, the method 100 includesthe step of placing 108 at least one mask between at least one radiationsource and the first and second radiation-sensitive materials. Incertain embodiments, the mask is provided as an integral part of theradiation source. A plurality of masks and a plurality of radiationsources may be employed. The mask has a plurality of substantiallyradiation-transparent apertures. In the embodiment also having the thirdradiation-sensitive material, for example, the mask is placed betweenthe at least one radiation source and the first, second, and thirdradiation-sensitive materials. It should be appreciated that a firstmask and a first radiation source may be disposed on a first side of thefirst and second radiation-sensitive materials, and a second mask and asecond radiation source may be disposed on a second side of the firstand second radiation-sensitive materials, as desired.

The material forming the mask may be a substantiallyradiation-transparent material, such as quartz glass in relation toultraviolet (UV) radiation, for example. The apertures may be holes orsubstantially radiation-transparent openings formed in an otherwiseopaque, radiation-blocking coating disposed on the substantiallyradiation-transparent mask material. In one illustrative embodiment, themask has a plurality of apertures with a diameter of about 10 microns.As further nonlimiting examples, the mask material may include one ofcrown glass, Pyrex glass, and a polyethylene terephthalate, such as aMylar® film. The mask may be lifted away after an exposure and cleanedfor reuse. Multiple masks with different patterns and types of theplurality of apertures may also be employed. The apertures may haveshapes that provide the radiation-cured elements with desiredcross-sectional shapes. For example, the apertures may be substantiallycircular to fabricate radiation-cured elements with ellipticalcross-sectional shapes. A skilled artisan may select suitable maskmaterials, aperture sizes and shapes, and resulting constructconfigurations, as desired.

The method 100 includes the step of exposing 110 the first and secondradiation-sensitive materials to a plurality of radiation beams. Theradiation beams are projected through the radiation-transparentapertures in the at least one mask and contact the first and secondradiation-sensitive materials. A skilled artisan may select theradiation source to generate electromagnetic radiation or particleradiation, as desired. The radiation beams employed to expose theradiation-sensitive material may be generated by a Mercury arc lampproviding UV radiation beams, for example. One of ordinary skill in theart understands that radiation beams of other wavelengths, such asinfrared, visible light, and X-ray radiation, and from other sources,such as incandescent lights and lasers, may also be employed. Particleradiation such as an electron beam from a cathode ray source may also beemployed. It should be further understood that the radiation beams maybe collimated, partially collimated, or non-collimated, as desired.

The plurality of radiation beams may include a plurality of firstradiation beams and a plurality of second radiation beams, for example.In certain embodiments, the first radiation beams may be different fromthe second radiation beams in at least one of frequency, amplitude, andtype. The first and second radiation-sensitive materials may be exposedto the plurality of radiation beams simultaneously or sequentially, asdesired. The first radiation beams may be different from the secondradiation beams in at least one of cross-sectional shape and angle ofincidence relative a surface of one of the first, second, and optionallythe third, radiation-sensitive materials. As a further nonlimitingexample, the plurality of first radiation beams is provided by a firstradiation source having a first mask and the plurality of secondradiation beams is provided by a second radiation source having a secondmask. Any desired variety of radiation frequencies, radiationamplitudes, radiation types, cross-sectional shapes, angles, masks, andradiation sources may be employed within the scope of the presentdisclosure.

The steps of applying 104, 106 the first and second radiation-sensitivematerials and exposing 110 the first and second radiation-sensitivematerials have been described hereinabove with respect to concurrentexposure of first and second radiation-sensitive materials. It should beappreciated that the first and second-radiation sensitive materials mayalso be exposed in a non-concurrent manner. For example, each of firstand second radiation-sensitive materials may be independently appliedand then exposed to the radiation beams. As a further example, the firstradiation-sensitive material may be first applied and then exposed tothe first radiation beams. The second radiation-sensitive material maythen applied to the first radiation-sensitive material. The secondradiation-sensitive materials may have a heightened sensitivity and beexposed to the second radiation beams for a duration which would notsubstantially affect the first radiation-sensitive material. In thismanner, construct features in the less sensitive firstradiation-sensitive material would not be carried through the moresensitive second radiation-sensitive material.

At least one substantially solid radiation-sensitive material may alsobe employed with at least one liquid radiation-sensitive material. Whereconcurrent exposure is desired, for example, the substantially solidfirst radiation-sensitive material may be applied to the substrate. Theliquid second radiation-sensitive material is then applied to thesubstantially solid first radiation-sensitive material. Each of thefirst and second radiation-sensitive materials may be selected to have adifferent sensitivity. If the variation in sensitivity between the firstand second radiation-sensitive materials is one of rate, then theconstruct features in the less sensitive radiation-sensitive materialwould be carried through the more sensitive radiation-sensitivematerial. The construct features in the more sensitiveradiation-sensitive material would not be carried through the lesssensitive radiation-sensitive material. If the variation in sensitivityis due to frequency or type of radiation, the construct features thatare unique to the first and second radiation-sensitive materials may begenerated separately. The construct features that cross the interfaceboundary between the first and second radiation-sensitive materials maybe formed by concurrently exposing the first and secondradiation-sensitive materials to both frequencies and/or and types ofradiation. For example, the first and second radiation-sensitivematerials may be exposed to provide less than a complete cure, theresidual uncured material may then be stripped away, and the first andsecond radiation-sensitive materials concurrently cured for a co-cure atthe interface boundary.

Where non-concurrent exposure is desired, the substantially solid firstradiation-sensitive material may be applied and then exposed to theradiation beams. The liquid second radiation-sensitive material is thenapplied to the substantially solid first radiation-sensitive material.Typically, the liquid second radiation-sensitive material may beselected to have the sensitivity greater than the sensitivity of thesubstantially solid first radiation-sensitive material. Constructfeatures formed in the liquid second radiation-sensitive material do notthereby carry through to the substantially solid firstradiation-sensitive material.

One of ordinary skill in the art should understand that the first andsecond radiation-sensitive materials may include liquid layers withvariable depths, for example. As a nonlimiting example, the liquidradiation-sensitive material may be applied to the substrate andsubsequently exposed to form desired construct features. Additionalliquid radiation-sensitive material may then be added to raise theheight of the original liquid radiation-sensitive material. Theadditional liquid radiation-sensitive material is exposed, and theintensity of the radiation beams and the exposure time controlled suchthat the new construct features only extend through the new layer of theliquid radiation-curable material. Alternatively, the intensity of theradiation beams and the exposure time may be controlled such that thenew construct features extend into the construct features formed in theoriginal liquid radiation-sensitive material.

Following the step of exposing 110 the first and second materials to theplurality of radiation beams, the method 100 includes the step offorming 112 the first and second constructs from the first and secondradiation-sensitive materials, respectively. It should be understoodthat the step of forming 112 the first and second radiation-sensitivematerials may directly result from the step of exposing 110 the firstand second radiation-sensitive materials to the plurality of radiationbeams. Alternatively, the step of forming 112 may further include apost-processing of the first radiation-sensitive material and the secondradiation-sensitive material following the step of exposing 110 thefirst and second radiation-sensitive materials to the plurality ofradiation beams. The post-processing may include a heating of the firstand second radiation-curable materials, for example. The heating mayfacilitate at least one of polymerization and crosslinking of at leastone of the first radiation-sensitive material and the secondradiation-sensitive material when at least one of the firstradiation-sensitive material and the second radiation-sensitive materialis a radiation-curable material that has undergone a degree ofinitiation by exposure to the radiation beams. In an alternativeembodiment, the heating may facilitate a dissociation of at least one ofthe first radiation-sensitive material and the secondradiation-sensitive material when at least one of the firstradiation-sensitive material and the second radiation-sensitive materialis a radiation-dissociable material that has undergone a degree ofdissociation by exposure to the radiation beams. Suitable temperaturesand heating times may be selected as desired.

The first radiation-sensitive material is exposed and forms a firstconstruct in the first radiation-sensitive material. The secondradiation-sensitive material is exposed and forms a second construct inthe second radiation-sensitive material. Where the thirdradiation-sensitive material is exposed to the radiation beams, thethird radiation-sensitive material forms a third construct. The firstconstruct, second construct, and optionally the third construct, areformed from a plurality of radiation-cured elements and cooperate toform the radiation-cured structure. It should be appreciated that avariety of the radiation-cured elements may be formed according to thepresent method, including truss elements, radiation-cured sheets, andsolid radiation-cured polymer structures, for example.

In a particular embodiment, the radiation-cured structure includes amicrotruss structure. The microtruss structure may have a plurality offirst truss elements that extend along a first direction, a plurality ofsecond truss elements that extend along a second direction, a pluralityof third truss elements that extend along a third direction, and aplurality of fourth truss elements that extend along a fourth direction.The first, second, third, and fourth truss elements may interpenetrateeach other at a plurality of nodes. It should be appreciated that thefirst, second, third, and fourth truss elements may not interpenetrateeach other, or may interpenetrate each other at the plurality of nodeson an intermittent basis, as desired. The first, second, third, andfourth truss elements form a continuous, three dimensional, selfsupporting cellular structure.

Although the microtruss structure with the plurality of first, second,third, and fourth truss elements may have a 4-fold architecturalsymmetry as described hereinabove, a skilled artisan should appreciatethat other architectures for the microtruss structure, such as a 3-foldsymmetry and a 6-fold symmetry, may be employed within the scope of thepresent disclosure. The particular architecture may be selected, forexample, to increase the microtruss structure connectivity and reducesusceptibility to bending and buckling of the microtruss structure undera load. The selected architecture may be symmetrical or asymmetrical, asdesired. The architecture may also be selected to optimize strength andstiffness of the microtruss structure. One of ordinary skill in the artshould further understand that the other architectures for microtrussstructure may be employed, as desired.

Exemplary architectures of the microtruss structure are described byJacobsen in U.S. Pat. No. 7,382,959 and U.S. patent application Ser. No.11/801,908. For example, the plurality of first truss elements may bedefined by a plurality of first self-propagating polymer trusswaveguides. The plurality of second truss elements may be defined by aplurality of second self-propagating polymer truss waveguides. Theplurality of third truss elements may be defined by a plurality of thirdself-propagating polymer truss waveguides. The plurality of fourth trusselements may be defined by a plurality of fourth self-propagatingpolymer truss waveguides. Other suitable means of forming the microtrussstructure may be employed as desired.

One of ordinary skill in the art should appreciate that the particularmicrotruss structure can be designed as desired, for example, by atleast one of: 1) selecting the angles and the patterns of the trusselements with respect to one another, 2) adjusting the packing, orrelative density of the resulting cellular structure, and 3) selectingthe cross-sectional shapes and dimensions of the truss elements. Inparticular, truss elements having an elliptical truss cross-sectionalshape may militate against degradation with differences in coefficientof thermal expansion. Other cross-sectional shapes may also be employed,as desired.

It should be appreciated that the radiation-cured structure according tothe present disclosure may be fabricated by the combined use ofradiation-curable materials and radiation-dissociable materials. Forexample, the radiation-cured structure may be formed from both of anegative resist and a positive resist as described hereinabove. Each ofthe negative and positive resists may be applied adjacent one another inan uncured state. The negative and positive resists may be selected tohave different radiation sensitivities. The negative resist may bephotoinitiated or at least partially cured during the step of exposing110 the negative resist to the radiation beams. Following the exposureof the negative resist to form the construct features, and the removalof residual uncured material, the uncured positive resist and thenegative resist may be co-cured. The negative resist construct featuresare thereby allowed to crosslink with the positive resist at theinterface boundary. The positive resist is then exposed to radiationbeams to dissociate the positive resist material in desired regionswithout affecting the construct formed in the negative resist. Theradiation-cured structure may thereby be formed from both of the firstradiation-curable material and the second radiation-dissociablematerial.

The method 100 of the present disclosure may further include the step ofremoving an uncured portion of the first and second radiation-sensitivematerials. It should be appreciated that the term “uncuredradiation-sensitive material” may also include dissociatedradiation-sensitive material within the scope of the present disclosure.The uncured portion may be a residual portion of the first and secondradiation-curable materials that was uncured following the exposure tothe radiation beams, or a portion of the first and secondradiation-dissociable materials that was scissioned or otherwise madesolvent-soluble by the exposure to the radiation beams. The step ofremoving the uncured portion typically occurs following at least one ofthe step of exposing 110 the first and second radiation-sensitivematerials to the plurality of radiation beams to cure the first andsecond radiation-sensitive materials. As a nonlimiting example, the stepof removing the uncured portion of the first and secondradiation-sensitive materials may include rinsing the radiation-curedstructure with the solvent. One of ordinary skill in the art shouldappreciate that suitable solvents do not substantially degrade theexposed radiation-cured structure during the step of removing theuncured portion of the first and second radiation-sensitive materials.

It should be understood that, following the fabrication of theradiation-cured structure, the radiation-cured structure may be furtherprocessed to enhance at least one of a strength, an electricalconductivity, and an environmental resistance thereof. As nonlimitingexamples, the method of the present disclosure may further include atleast one of the steps of metalizing, carbonizing, and ceramicizing theradiation-cured structure. Composites including the radiation-curedstructure may also be formed, for example, as disclosed in U.S.application Ser. No. 12/008,479 to Jacobsen et al., hereby incorporatedherein by reference in its entirety.

In one embodiment, the radiation-cured structure may be metalized byplating the radiation-cured elements with a metal coating. The metalcoating may be substantially oxidation resistant, reduction resistant,and acid-resistant, for example. The metal coating may include a noblemetal selected from the group consisting of: ruthenium (Ru), rhodium(Rh), palladium (Pd), silver (Ag), iridium (Ir), platinum (Pt), andosmium (Os), and alloys thereof, for example. In a particularembodiment, the metal coating is gold (Au). In another particularembodiment, the metal coating is tantalum (Ta). Another suitable metalcoating may include nickel (Ni) alloys, such as alloys of nickel (Ni)and chromium (Cr) or nickel (Ni) and cobalt (Co). As should berecognized by one of ordinary skill in the art, the metal coating mayinclude mixtures or alloys of the above identified metals. Otherelectrically conductive metals and materials may also be employed, asdesired.

The metal coating may be deposited onto the radiation-cured structure byat least one of electron beam evaporation, magnetron sputtering,physical vapor deposition, chemical vapor deposition, atomic layerdeposition, electrolytic deposition, electroless deposition, flame spraydeposition, brush plating, and other like processes. Solution basedelectroplating techniques that include immersing the radiation-curedstructure in a plating bath may also be employed. Application of metalin the form of a slurry powder and subsequently firing the slurry powderto form the metal coating may also be used. A skilled artisan may selectmore than one deposition technique to take into account differencesbetween line of sight and non-line of sight characteristics of thedeposition techniques selected. In certain embodiments, the metalcoating may be substantially evenly deposited on both the interior andexterior surfaces of the radiation-cured structure. Suitable means formetalizing the radiation-cured structure may be selected as desired.

One of ordinary skill in the art should understand that theradiation-cured structure may be carbonized. The carbonization of theradiation-cured structure may cause the radiation-cured structure tobecome electrically conductive. Open-cellular carbon structures and amethod of making the same from a polymer template material is disclosedby Jacobsen in U.S. patent application Ser. No. 11/870,379, thedisclosure of which is hereby incorporated herein by reference in itsentirety. Other suitable methods for carbonizing the radiation-curedstructure may also be used.

It should be appreciated that the radiation-cured structure may beceramicized by coating the radiation-cured structure with a suitablemetal oxide or ceramic. In certain illustrative embodiments, at least aportion of the radiation-cured structure may be coated with the metaloxide or the ceramic to provide the desirable level of bending strengthand electrical conductivity. Suitable ceramic structures and methods forceramicizing radiation-cured structures are disclosed by Gross et al. inU.S. patent application Ser. No. 12/074,727, the entire disclosure ofwhich is hereby incorporated herein by reference. Other suitable methodsfor ceramicizing the radiation-cured structure may also be used.

The method 100 of the present disclosure may include the step ofapplying a filter layer between the first radiation-sensitive materialand the second radiation-sensitive material. The filter layer may be alayer of another radiation-sensitive material that is substantiallyopaque to the radiation to which the first and secondradiation-sensitive materials are sensitive. Illustratively, the filterlayer militates against an exposure of the second radiation-sensitivematerial to at least a portion of the plurality of radiation beams whenthe first radiation-sensitive material is exposed. In particularinstances, the filter layer is an optical filter layer. For example, thefilter layer may be disposed between the first radiation-sensitivematerial and the second radiation-sensitive material. The filter layermay thereby militate against radiation beams of select frequency,amplitude, or type from causing a formation of the first construct inthe first radiation-sensitive material when the secondradiation-sensitive material is exposed to the radiation beams from theradiation source. The filter layer may also have a radiation sensitivitydifferent from the sensitivity of the first and secondradiation-sensitive materials. Therefore, another construct of theradiation-cured structure may also be formed in the filter layer, asdesired.

The embodiments of the invention shown in FIGS. 2 and 3 are similar tothe embodiment shown in FIG. 1 except as described below. For purpose ofclarity, like steps from FIG. 1 shown in FIGS. 2 and 3 are repeated withthe same reference numerals in the 200s and 300s instead of the 100s.

As shown in FIG. 2, the first radiation-curable material has the firstsensitivity and the second radiation-curable material has the secondsensitivity different from the first sensitivity. The method 200includes the steps of providing 202 the substrate; applying 204 thefirst radiation-curable material to the substrate; applying 206 thesecond radiation-curable material to the first radiation-curablematerial; placing 208 at least one mask between the at least oneradiation source and the first and second radiation-curable materials;and exposing 210 the first and second radiation-curable materials to theplurality of radiation beams through the radiation-transparent aperturesin the at least one mask. The method 200 includes the step of curing 212the first and second radiation-curable materials having the differentfirst and second sensitivities. The first construct in the firstradiation-curable material and the second construct in the secondradiation-curable material are thereby fabricated and cooperate to formthe radiation-cured structure. It should be further appreciated that thefirst radiation-curable material may be at least partially uncured atthe interface boundary with the second radiation-curable material priorto the curing 212 of the second radiation-curable material. Following astripping away of uncured radiation-curable material, the first andsecond radiation-curable materials may be concurrently cured to generatebonds at the interface boundary. The at least partial undercure of thefirst radiation-curable material prior to the curing 212 of the secondradiation-curable material may improve the adhesion of the firstconstruct to the second construct. It should be further appreciated thatthe difference in sensitivities between the first radiation-curablematerial and the second radiation-curable material allows the firstconstruct in the first radiation-sensitive material to be formed beforethe second construct in the second radiation-sensitive material, orvice-versa, as desired.

In an illustrative example, the first and second radiation-curablematerials are selected to have different sensitivities to radiationfrequency. In a particular embodiment, the first and secondradiation-curable materials are photoinitiated polymers. The first andsecond radiation-curable materials may be applied together and each havea sensitivity to a different frequency of radiation, for example Thefirst and second radiation-curable materials may be exposed to thedifferent frequencies of radiation either simultaneously or inprogression. After the features of the first and second constructs areinitiated within the first and second radiation-curable materials, forexample, each of the first and second radiation-curable materials may besimultaneously heated to the desired curing temperature. The heating maycause at least one of the polymerization and the crosslinking of thefirst and second constructs to proceed. Since each of the first andsecond constructs are cured at the same time, the construct featureswhich cross the interface between the first and second radiation-curablematerials are able to carry the polymerization or crosslinking acrossthe interface boundary. A desirable level of adhesion between the firstand second constructs may thereby be provided.

In another embodiment, the first and second radiation-curable materialsare photocured polymers. If all of the construct features for each ofthe first and second radiation-curable materials are exposed atsubstantially the same time, the reactions in each of the first andsecond radiation-curable materials occur substantially simultaneouslyand the polymerization reactions is carried across the interfaceboundaries between the first and second radiation-curable materials.Where the construct features cannot all be exposed at the same time dueto limitations of the masking or radiation sources, for example, theconstruct features in each of the first and second radiation-curablematerials that do not tie into the other of the first and secondradiation-curable materials may be formed separately. The constructfeatures which do cross the boundary may be concurrently formed in thefirst and second radiation-curable materials on each side of theboundary. It should be appreciated that these restrictions may not be assevere if one of the first and second radiation-curable materials doesnot fully cure during the exposure process, for example, as describedhereinabove.

In another illustrative example, the first and second radiation-curablematerials are selected to have different initiation rates. It should beunderstood that the photoinitiation process of the present method 200may be a function of at least one of the radiation intensity andexposure time. Either may be varied to change the exposure. Features ofthe construct formed in one of the first and second radiation-curablematerials with a slower initiation rate, for example, are desirablycarried into the adjacent one of the first and second radiation-curablematerials with the faster initiation rate. Additional construct featuresmay be added to the one of the first and second radiation-curablematerials with the faster initiation rate, for example. Following theinitiation process, the first and second constructs may be cured byincreasing the temperature of the first and second radiation-curablematerials. In order to allow construct features to cross the interfaceboundaries, it should be understood that all features must be initiatedprior to the beginning of the curing process.

In a further illustrative example, the first and secondradiation-curable materials are selected to have a different cure rate.The processing of the first and second radiation-curable materials withthe different cure rate may be very similar to processing of the firstand second radiation-curable materials with the different initiationrate. It should be appreciated that the curing of the first and secondconstructs happens during exposure. Like effects with regard to speed ofthe curing apply, however. The construct features formed in one of thefirst and second radiation-curable materials with the slower cure rateare desirably carried through the one of the first and secondradiation-curable materials with the faster cure rate, for example. Theone of the first and second radiation-curable materials with the fastercuring rate may contain additional features, as desired.

In an additional illustrative example, the first and secondradiation-curable materials are selected to have a different sensitivityto the type of radiation. As described hereinabove, a multitude ofradiation types may be use to initiate the curing reaction. Typically,the curing (e.g., at least one of initiation, polymerization, andcrosslinking) will be initiated by some form of electromagneticradiation. In addition, however, other types of radiation such asparticle beam radiation can also be used to initiate the curing.

For a photoinitiated polymer system, where the first and secondradiation-curable materials also have different sensitivities to theradiation type, the process is similar to the process for varying thefrequency of the radiation. The first and second radiation-curablematerials are each applied and have the sensitivity to the differenttype of radiation. The first and second-radiation-curable materials maythen be exposed to the different kinds of radiation, eithersimultaneously or in progression. After the construct features withinthe first and second radiation-curable materials are initiated, each ofthe first and second radiation-curable material are simultaneouslyheated to the curing temperature where polymerization and/orcrosslinking of all construct features occur. Since the constructfeatures are formed at substantially the same time, the constructfeatures which cross the interface boundary between the first and secondradiation-curable materials desirably carry the polymerization and/orcrosslinking across the interface boundary.

For a photocured polymer system, where each of the construct featuresfor the first and second radiation-curable materials can be exposed atsubstantially the same time, the curing in the first and secondradiation-curable materials can occur simultaneously. The resultingpolymerization reactions can thereby be carried across the interfaceboundary between the first and second radiation-curable materials. Wherethe construct features cannot all be exposed at substantially the sametime due to limitations of the masking or radiation sources, forexample, the construct features in the first radiation-curable materialthat do not tie into the second radiation-curable material may be formedseparately. The construct features which do cross the interface boundarymay be concurrently formed in the first and second radiation-curablematerials on each side of the interface boundary.

As shown in FIG. 3, the first radiation-dissociable material has a firstsensitivity and the second radiation-dissociable material has a secondsensitivity different from the first sensitivity. The method 300includes the steps of providing 302 a substrate; applying 304 the firstradiation-dissociable material to the substrate; applying 306 the secondradiation-dissociable material to the first radiation-dissociablematerial; placing 308 at least one mask between the at least oneradiation source and the first and second radiation-dissociablematerials; and exposing 310 the first and second radiation-dissociablematerials to the plurality of radiation beams through theradiation-transparent apertures in the at least one mask. The method 300further includes the step of dissociating 312 the first and secondradiation-dissociable materials having the different first and secondsensitivities. The first construct in the first radiation-dissociablematerial and the second construct in the second radiation-dissociablematerial are thereby formed and cooperate to form the radiation-curedstructure. It should be appreciated that the difference in sensitivitiesbetween the first radiation-dissociable material and the secondradiation-dissociable material allows the first construct in the firstradiation-sensitive material to be formed before the second construct inthe second radiation-sensitive material, or vice-versa, as desired.

In an illustrative example, the first and second radiation-dissociablematerials are selected to have a different sensitivity to radiationfrequency. Where the first and second radiation-dissociable materialsare positive resists, for example, the first and secondradiation-dissociable materials desirably are fully cured andcrosslinked together prior to the step of exposing 310 the first andsecond radiation-dissociable materials to the radiation beams. Thedesired construct features may thereby be carried across interfaceboundary between the first and second radiation-dissociable materials.It should be understood that the first and second radiation-dissociablematerials may be exposed to the radiation beams having the differentfrequencies in any order, and then the exposed regions can be dissolvedby the solvent to leave the desired first and second constructs.

In another illustrative example, the first and secondradiation-dissociable materials are selected to have differentdissociations rate. It should be appreciated that, where the rates ofdissociation of the first and second radiation-dissociable materials aredifferent, complex constructs can be generated. One of ordinary skill inthe art understands that the holes generated by the step of exposing 310one of the first and second radiation-dissociable materials with theslowest dissociation rate will be carried through the entire resultingradiation-cured structure. Additional material may be removed from theone of the first and second radiation-dissociable materials with thefaster dissociation rate without significantly affecting the other ofthe first and second radiation-dissociable materials with the slowerdissociation rate. However, a significant difference in the respectivedissociation rates is generally desirable. As nonlimiting examples,where there are only two radiation-dissociable materials, a differencein dissociation rates of about 10:1 may be sufficient; where there arethree radiation-dissociable materials, a different in dissociation ratesof about 100:1 may be sufficient; and where there are more than threeradiation-dissociable materials, a difference in dissociation rates ofabout 10,000:1 may be sufficient. The respective dissociation rates ofthe radiation-dissociable materials may be selected, as desired.

In a further illustrative example, the first and secondradiation-dissociable materials may be selected to have differentsensitivities to the type of radiation. The forming of theradiation-cured structure from the first and secondradiation-dissociable materials with sensitivities to different types ofradiation may be similar to the method for different sensitivity tofrequency, described hereinabove. As a nonlimiting example, the firstradiation-dissociable material may be sensitive to electromagneticradiation and the second radiation-dissociable material may be sensitiveto particle beam radiation. It should be appreciated, however, that thefirst radiation-dissociable material that is sensitive to theelectromagnetic radiation is desirably selected to have minimalsensitivity to the particle beam radiation, and vice versa.

It is surprisingly found that the selective exposure of laminatedradiation-sensitive materials, according to the methods 100, 200, 300 ofthe present disclosure, minimizes production costs and fabrication timeof radiation-cured structures. By grouping all the lamination operationstogether, the fabrication of complex radiation-cured structures can nowbe performed on a same line and in a same facility. Exposing thedifferent radiation-sensitive materials to the plurality of radiationbeams can be conducted by turning masked radiation sources on withoutmoving the radiation-sensitive materials. The methods 100, 200, 300thereby militate against misalignment and tolerance concerns thattypically may result from repeated positioning of masks andradiation-sensitive materials. The radiation-cured structure of thepresent disclosure may also be fabricated without the need forplanarizing, as is performed in electromechanical fabrication. A singlecleaning operation may also be employed to remove uncuredradiation-sensitive materials following fabrication of theradiation-cured structure. Likewise, single metallization,carbonization, and ceramicization operations may be used to make theradiation-cured structure to provide the radiation-cured structure withdesired characteristics.

While certain representative embodiments and details have been shown forpurposes of illustrating the invention, it will be apparent to thoseskilled in the art that various changes may be made without departingfrom the scope of the disclosure, which is further described in thefollowing appended claims.

1. A method for fabricating a radiation-cured structure, the methodcomprising the steps of providing a first radiation-sensitive material,the first radiation-sensitive material having a first sensitivity;applying a second radiation-sensitive material to the firstradiation-sensitive material, the second radiation-sensitive materialhaving a second sensitivity different from the first sensitivity;placing at least one mask between at least one radiation source and thefirst and second radiation-sensitive materials, the mask having aplurality of substantially radiation-transparent apertures; exposing thefirst and second radiation-sensitive materials to a plurality ofradiation beams through the radiation-transparent apertures in the mask;and forming at least a first construct in the first radiation-sensitivematerial and a second construct in the second radiation-sensitivematerial, the first construct different from the second construct, thefirst construct and the second construct cooperating to form theradiation-cured structure.
 2. The method of claim 1, wherein at leastone of the first and second radiation-sensitive materials is one of aradiation-curable material and a radiation-dissociable material.
 3. Themethod of claim 1, wherein at least one of the first and secondsensitivities is at least one of a curing rate, an initiation rate, adissociation rate, a sensitivity to radiation frequency, a sensitivityto radiation amplitude, a sensitivity to radiation type.
 4. The methodof claim 1, further comprising the step of removing an uncured portionof the first and second radiation-sensitive materials following the stepof exposing the first and second radiation-sensitive materials to theplurality of radiation beams.
 5. The method of claim 1, furthercomprising the step of applying a third radiation-sensitive material tothe second radiation sensitive material, the third radiation-sensitivematerial having a third sensitivity.
 6. The method of claim 5, whereinthe third radiation-sensitive material forms a third construct followingthe step of exposing the third radiation-sensitive material to theplurality of radiation beams, the third construct cooperating with thefirst construct and the second construct to form the radiation-curedstructure
 7. The method of claim 5, wherein the third sensitivity issubstantially the same as the first sensitivity.
 8. The method of claim5, wherein the third sensitivity is different from the first sensitivityand the second sensitivity.
 9. The method of claim 1, further comprisingthe steps of providing a substrate and applying the firstradiation-sensitive material to the substrate.
 10. The method of claim1, further comprising at least one of the steps of metalizing,carbonizing, and ceramicizing the radiation-cured structure.
 11. Themethod of claim 1, wherein the plurality of radiation beams include aplurality of first radiation beams and a plurality of second radiationbeams.
 12. The method of claim 11, wherein the plurality of firstradiation beams is different from the plurality of second radiationbeams in at least one of frequency, amplitude, and type.
 13. The methodof claim 11, wherein the plurality of first radiation beams is differentfrom the plurality of second radiation beams in at least one ofcross-sectional shape and angle of incidence relative a surface of oneof the first radiation-sensitive material and the secondradiation-sensitive material.
 14. The method of claim 13, wherein theplurality of first radiation beams is provided by a first radiationsource having a first mask and the plurality of second radiation beamsis provided by a second radiation source having a second mask.
 15. Themethod of claim 14, wherein at least one of the firstradiation-sensitive material and the second radiation-sensitive materialis one of a negative resist and a positive resist.
 16. The method ofclaim 14, wherein at least one of the first radiation-sensitive materialand the second radiation-sensitive material is a liquid photomonomer.17. The method of claim 14, wherein the forming step includes the stepof heating the first radiation-sensitive material and the secondradiation-sensitive material following the step of exposing theradiation-sensitive material to the plurality of radiation beams, theheating facilitating at least one of polymerization, a crosslinking, anda dissociation of at least one of the first radiation-sensitive materialand the second radiation-sensitive material.
 18. The method of claim 1,further comprising the step of applying a filter layer between the firstradiation-sensitive material and the second radiation-sensitivematerial, the filter layer militating against an exposure of one of thefirst and second radiation-sensitive materials to at least a portion ofthe plurality of radiation beams.
 19. A method for fabricating aradiation-cured structure, the method comprising the steps of: providinga first radiation-curable material, the first radiation-curable materialhaving a first sensitivity including at least one of a first curingrate, a first initiation rate, a sensitivity to a first radiationfrequency, a sensitivity to a first radiation amplitude, and asensitivity to a first radiation type; applying a secondradiation-curable material to the first radiation-curable material, thesecond radiation-curable material having a second sensitivity includingat least one of a second curing rate, a second initiation rate, asensitivity to a second radiation frequency, a sensitivity to a secondradiation amplitude, and a sensitivity to a second radiation type, thesecond sensitivity different from the first sensitivity; placing atleast one mask between at least one radiation source and the first andsecond radiation-curable materials, the mask having a plurality ofsubstantially radiation-transparent apertures; exposing the first andsecond radiation-curable materials to a plurality of radiation beamsthrough the radiation-transparent apertures in the mask; and forming atleast a first construct in the first radiation-curable material and asecond construct in the second radiation-curable material, the firstconstruct different from the second construct, the first construct andthe second construct cooperating to form the radiation-cured structure.20. A method for fabricating a radiation-cured structure, the methodcomprising the steps of: providing a first radiation-dissociablematerial, the first radiation-dissociable material having a firstsensitivity including at least one of a first dissociation rate, asensitivity to a first radiation frequency, a sensitivity to a firstradiation amplitude, and a sensitivity to a first radiation type;applying a second radiation-dissociable material to the firstradiation-dissociable material, the second radiation-dissociablematerial having a second sensitivity including at least one of a seconddissociation rate, a sensitivity to a second radiation frequency, asensitivity to a second radiation amplitude, and a sensitivity to asecond radiation type, the second sensitivity different from the firstsensitivity; placing a mask between a radiation source and the first andsecond radiation-dissociable materials, the mask having a plurality ofsubstantially radiation-transparent apertures; exposing the first andsecond radiation-dissociable materials to a plurality of radiation beamsthrough the radiation-transparent apertures in the mask; and forming atleast a first construct in the first radiation-dissociable material anda second construct in the second radiation-dissociable material, thefirst construct different from the second construct, the first constructand the second construct cooperating to form the radiation-curedstructure.